This invention relates to a process for the manufacture of silyl ketene acetals (SKA). More specifically, this invention relates to a process for producing silyl ketene acetals from the reaction of esters of alpha-substituted carboxylic acids with an alkali metal in the presence of an organohalosilane.
For the purposes of the instant invention, esters of alpha-substituted carboxylic acids will be referred to as "alpha-esters"; as a further example, esters of carboxylic acids substituted in the alpha-position with halogen will be referred to as "alpha-haloesters." Likewise, esters of beta-substituted carboxylic acids will be referred to as "beta-esters."
The first reference to preparation of silyl ketene acetals (SKA) was in the late-1950's by Petrov et al., J. Gen. Chem. (USSR), 29(1959), pp. 2896-2899. This reference and most of the other references to the art deal with chemical species of the general formula, EQU (CH.sub.3).sub.2 C.dbd.C[O(CH.sub.2).sub.v Z](OSiR.sub.3).
R is selected from a group consisting of alkyl groups, aryl groups, alkaryl groups, and substituted alkyl, aryl, and alkaryl groups; v has a value of 0, 1 or more; Z is such groups as alkyl, alkenyl, aryl, alkaryl; any of these groups containing one or more functional groups, such as ether oxygen atoms, thio groups, organosiloxy groups, which are unreactive under silylating conditions. These organosilane intermediates are of value because of the ability to further react the SKA to prepare organic compounds which would be difficult to synthesize by other means. A very recent application is the use of the SKA as acrylate polymerization initiators. This concept known as Group Transfer Polymerization (GTP) was developed by DuPont and is disclosed in three U.S. patents--U.S. Pat. No. 4,414,372, Farnham et al., issued Nov. 8, 1983; U.S. Pat. No. 4,417,034, Webster, issued Nov. 22, 1983; and U.S. Pat. No. 4,508,880, Webster, issued Apr. 2, 1985.
Four procedures for preparing silyl ketene acetals are known in the art. The first general route to SKA is the reaction of an ester of a carboxylic acid with an appropriate metal reagent to form a metal enolate ion and subsequent reaction of the enolate ion with an organochlorosilane. Ainsworth et al., J. Orqanometallic Chem., 46(1972), pp. 59-71, describe the preparation of an SKA via the reaction of esters of carboxylic acids with lithium diisopropylamide, followed by reaction with trimethylchlorosilane, Kita et al., Tetrahedron Letters, 24:12 (1983), pp. 1273-1276, discloses a similar procedure to prepare bifunctional SKA, Brown, J. Org. Chem., 39:9(1974), pp. 1324-1325, describes the preparation of metal enolate ions by reacting potassium hydride in tetrahydrofuran with a carbonyl compound, followed by reaction with excess triethylamine and trimethylchlorosilane.
Kuo et al., Chemical Communications, (1971), pp. 136-137, discloses the preparation of silyl ketene acetals of the formula, EQU R.sup.1 R.sup.2 C.dbd.C[OSi(CH.sub.3).sub.3 ].sub.2,
wherein R.sup.1 and R.sup.2 are hydrogen, methyl, t-butyl, and phenyl, The silyl ketene acetal is prepared by the reaction of the corresponding carboxylic acid or silyl ester of a carboxylic acid in contact with lithium diisopropylamide, trimethylchlorosilane, and tetrahydrofuran. Yields of the desired silyl ketene acetal of from 29 to 85 percent are disclosed. Kuo et al., are silent as to whether or not the yield figures disclosed are calculated by analysis or physical isolation and separation.
In a second general procedure, silyl ketene acetals are prepared by the hydrosilation of esters of carboxylic acid with organohydrosilanes. Petrov et al., J. Gen. Chem. (USSR), 29(1959), pp. 2896-2899, described the platinumcatalyzed reaction of methyl methacrylate with triethylsilane. Ojima et al., J. Organometallic Chem., 111(1976), pp. 43-60, studied the use of tris(triphenylphosphine)rhodium chloride as a catalyst. Howe et al., J. Organometallic Chem., 208(1981), pp. 401-406, and Yoshii et al., Chem. Pharm. Bull., 22(1974), pp. 2767-2769, describe yields of 70-75% SKA from the reaction of (C.sub.2 H.sub.5).sub.3 SiH and methyl methacrylate using organophosphorous complexes of rhodium as a catalyst. Quirk et al., in European Patent Application No. 0184692, published June 18, 1986, discloses o-silylated ketene acetals and enol ethers and a process for their preparation from the reaction of acrylate esters and silanes or siloxanes in the presence of a rhodium catalyst.
In a third procedure Ishikawa et al., in U.S. Pat No. 4,482,729, issued Nov. 13, 1984, describes the preparation of a fluoroalkyl silyl ketene acetal by the reaction of a fluorinated carboxylic acid ester with trimethylsilyl trifluoromethanesulfonate.
The fourth procedure involves the alkali metal reduction of disubstituted malonates in the presence of trimethylchlorosilane to produce a silyl ketene acetal, Kuo et al., Chemical Communications, (1971), pp. 136-137; and J. Am. Chem. Soc., 94:11 (1972), pp. 4037-4038, disclose the preparation of silyl ketene acetals of the formula, EQU R.sup.1 R.sup.2 C.dbd.C(OR.sup.3)OSi(CH.sub.3).sub.3,
from the reaction of a dialkyl dialkylmalonate with trimethylchlorosilane in the presence of sodium metal, wherein the R.sup.1 and R.sup.2 are methyl; ethyl, or phenyl; and R.sup.3 is methyl or ethyl.
Ruhlmann, Synthesis (1971), pp. 236-253, particularly Section 1.1.5.2, discusses the reactions of alpha- and beta-haloesters of carboxylic acids with sodium and trimethylchlorosilane, Ruhlmann points out that the primary reaction of the beta-haloesters is the conversion of the ester to their corresponding silyl cyclopropanone ketals, the betahaloesters having the formula, ##STR3## and the ketals having the formula, ##STR4## Ruhlmann shows that the alpha-haloesters, ##STR5## follow a more complicated route. Ruhlmann points out that esters such as methyl chloroacetate yield almost exclusively the C-silylated ester, ##STR6## which undergoes acyloin condensation to yield tetrasilylated products--in the case of methyl chloroacetate, the product is 1,4-bis(trimethylsilyl)-2,3-bis(trimethylsilyloxy)-2-butene. Only in one example did the author cite a case in which the predominant product was a silyl ketene acetal. This case was the reaction of ethyl 2-bromopropionate with trimethylchlorosilane and sodium in diethyl ether. The reported yield was 75% silyl ketene acetal. Ruhlmann's example involved an ester in which the halogen atom is bonded to a secondary carbon atom, and not a tertiary carbon atom as taught by the instant invention. Ruhlmann's example was carried out in the examples, infra. The results of this example indicated that while a silyl ketene acetal was formed, there were also substantial quantities of both the C-silylated ester and the acyloin condensation product. The inventors have found unexpectedly that alpha-esters in which a halogen atom or alkoxy group is bound to a tertiary carbon atom react with alkali metals in the presence of excess organohalosilanes to yield silyl ketene acetals almost exclusively. There is virtually no acyloin condensation product.
Ruhlmann et al., J. Organometal. Chem., 27(1971), pp. 327-332, shows that a minor amount of silyl ketene acetals could be produced from the reaction of alkyl phenylacetates with sodium in the presence of trimethylchlorosilane. The yields of silyl ketene acetal were low, 2-8%; benzylsilanes and bissiloxyalkenes were the primary products.
Nowhere does prior art demonstrate or suggest the general preparation of silyl ketene acetals in high yields from the reaction of an alkali metal and an alpha-ester of a carboxylic acid, an ester in which a halogen atom or an alkoxy group is bonded to a tertiary carbon atom, in the presence of an organohalosilane.